In multithreaded programming, it’s not uncommon to encounter situations where one wants to poll on a global flag until it becomes the desired value. The straightforward way is to write something like:

1
while (flag != expected) ;

The problem with it is that after turning on optimization, it may become a infinite loop, for the compiler can’t see the variable is change in the loop body. One way to work around it is to use cpu_relax(), namely memory clobber in assembly, which tells the compiler to relinquish the assumption that x is kept intact, and reload it each iteration. In addition, the compiler is smart enough to only flush global visible variables.

Let’s illustrate it by view the generated assembly code for the following C code. Assembly is obtained using clang -S -O test.c.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
#include <stdint.h>
#include <stdio.h>

unsigned int x;

#define cpu_relax() asm volatile("pause" ::: "memory")

void f()
{
  // unsigned int y;
  static unsigned int y;
  while (x == 0) {
    y += 3;
    cpu_relax();
    ;
  }
  x = y;
}

int main(int argc, char *argv[])
{
  return 0;
}

In this case, y is a local variable, so it’s kept in %eax without being flushed to memory.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
...
# local variable
f:                                      # @f
	.cfi_startproc
# BB#0:
	cmpl	$0, x(%rip)
                                        # implicit-def: %EAX
	jne	.LBB0_2
	.p2align	4, 0x90
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
	addl	$3, %eax
	#APP
	pause
	#NO_APP
	cmpl	$0, x(%rip)
	je	.LBB0_1
.LBB0_2:
	movl	%eax, x(%rip)
	retq
...

In this case, y is static variable, whose storage if static, so its value needs to be flushed and re-read around the barrier.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
...
# static local variable
f:                                      # @f
	.cfi_startproc
# BB#0:
	movl	f.y(%rip), %eax
	cmpl	$0, x(%rip)
	jne	.LBB0_2
	.p2align	4, 0x90
.LBB0_1:                                # =>This Inner Loop Header: Depth=1
	addl	$3, %eax
	movl	%eax, f.y(%rip)
	#APP
	pause
	#NO_APP
	cmpl	$0, x(%rip)
	movl	f.y(%rip), %eax
	je	.LBB0_1
.LBB0_2:
	movl	%eax, x(%rip)
	retq
...

§Reference